Final answer:
Nuclear power has the best safety record among the major sources of energy.
Step-by-step explanation:
Among the major sources of energy listed, nuclear has the best safety record. According to the provided information, nuclear power has one of the lowest levels of fatalities per unit of energy generated compared to other energy sources. Coal, petroleum, natural gas, and hydroelectricity have caused more fatalities per unit of energy due to air pollution and accidents. Switching from coal to nuclear power has prevented millions of air pollution-related deaths and reduced carbon dioxide emissions significantly.